What is transfer path analysis

Transcription

1 Siemens PLM Software What is transfer path analysis I. Introduction Transfer path analysis (TPA) is a test-based or simulation-based procedure which allows you to trace the flow of vibro-acoustic energy from a source, through a set of known structure- and air-borne transfer pathways, to a given receiver location. Figure 1 schematically shows the impact of a source to an operator. Figure 1: Transfer path analysis quantifies and visualizes the strengths of selected sources and their contribution via multiple transmission paths to a selected receiver signal The goal is to evaluate the contribution along each transfer path from the source to the receiver, so that one can identify the components along that path that need to be modified to solve a specific problem and perhaps to optimize the design by choosing desirable characteristics for these components. Performing a transfer path analysis is a worthwhile exercise, because the insights it gives lead directly to faster troubleshooting, better product refinement and a more methodical approach to vibro-acoustic design. Technical info issued by: Siemens. 1

2 TPA is just one step in the NVH optimization process. The first step in this process is the problem identification. It consists of performing preliminary measurements aimed at defining optimal test conditions for data acquisition and at identifying critical frequency ranges of the noise spectra. This step is followed by an operational deflection shape analysis. Once this has been realized, a full TPA analysis can be performed on the identified possible paths. After the TPA is complete, structural modal analysis or acoustic modal analysis help find root causes of possible noise and vibration issues and solve them. TPA can be applied to solve vibro-acoustics issues in many manufacturing industries. Performing TPA on a car engine helps reduce interior noise, driving wheel and seats vibration and thus improve driver s and passenger s comfort. Road noise disturbances in a vehicle can be minimized using the multireference TPA technique. Nowadays, TPA is even employed to complement pass-by noise engineering methods such as to reduce the overall vehicle s pass-by noise. TPA methods have also successfully been applied to printers to reduce sound power levels, as well as to household appliances such as refrigerators, washing machines or dish washers. TPA is a systematic method that can be used for all types of structures, large or small, such as boats and ships, wind turbines or even full aircrafts. The methodology of transfer path analysis was developed as a result of research carried out in the eighties and the beginning of the nineties. The first TPA product was industrialized in the early nineties on the LMS CADA-X platform. 20 years later, the methods are used in manufacturing industries worldwide, amongst which most major Automotive OEMs. With the broadest portfolio of TPA solutions on the market, LMS Test.Lab helps customers tackle issues from every possible angle from simple systems to complex structures. LMS Test.Lab TPA features a wide range of methods, such as LMS Test.Lab Single Reference Transfer Path Analysis and Airborne Source Quantification, LMS Test.Lab Multi-reference Transfer Path Analysis, LMS Test.Lab OPAX Operational Transfer Path Analysis and LMS Test.Lab Time Domain TPA. II. What is transfer path analysis In complicated structures involving many sub-assemblies (such as an automobile, aircraft or submarine) the vibro-acoustic sensations that are experienced by an observer at any one location may easily have been caused by a vibration source some way off. For example, the energy from a source in a car is transmitted into the passenger cavity by a number of different routes: from the engine mountings, the exhaust system connection points and indirectly even via the drive shafts and the wheel suspension. Airborne contributions from the intake or exhaust system, for instance, may be important as well. Some of these paths are important, some are not. Some transfer paths may cause interference at certain frequencies such that the observer does not notice anything significant until he moves position. Transfer path analysis is used to assess the structure- and airborne energy paths between excitation source(s) and receiver location(s). This is reflected is figure 2 which descriobes the so-called Source-Transfer-Receiver model, where an operational force (i.e. an engine) is multiplied with a transfer function leading to response at the receiver location (i.e. driver s ear). Transfer path analysis is a systematic method to understand the relation between multiple sources of noise and vibration and their effect on perceived user comfort and health. Transfer path analysis creates a physical mathematical model of the structure under test that allows: The quantification of source strengths The ranking of contributions of the different sources along the different paths for every target receiver positions What-if simulation of design changes by modifying the forces with new connection stiffness or changing transfer functions by eliminating resonances. Figure 2: The Source-Transfer-Receiver model Technical info issued by: Siemens. 2

3 The comparison of effects of various design modifications can be made in a TPA model. The design work can then focus on the most promising modifications. If we take the example of a vehicle, a typical approach is to analyze the interior acoustic response both objectively and subjectively, to identify the disturbing spectral components or even the lack of masking constituents. TPA allows quantifying the contributions of the various sources and their paths: Which is important? Which contribute? And which have a cancelling effect? TPA is then used to assess the structure-borne and airborne energy transfer routes from the source of excitation to a given receiver location. Once the sources and their paths have been quantified and modeled the root causes can be identified and engineers can focus on optimizing the system. Typical improvements are hardware modifications of suspension elements or body parts; local impedance changes of body connection points, or, in the later stages of development, tuned vibration absorbers. 3. Design Transfer path analysis can be used in combinations of hybrid FE/experimental modeling for acoustic optimization of a design. It also helps in the cascading of overall assembly NVH targets into individually specified subassemblies. In principle, the designer can substitute different components into the various paths and use a sound quality replay system to listen to the effects subjectively even during the early design stages. However, while such virtual prototyping is a worthwhile goal, the current state-of-the-art generally allows such approaches to be used for trend analysis rather than for bypassing a physical prototype stage altogether. IV. What do the results of a transfer path analysis look like? A transfer path analysis generates a large amount of data. A workflow-oriented tool with a powerful data management helps engineers continuously checks data and minimizes translation issues and operator errors. Users need to perform data processing and results interpretation fast and efficiently. III. When is transfer path analysis performed? Transfer path analysis is a powerful tool that can be applied at several distinct stages of a vehicle or a machine development. 1. Troubleshooting Problems often arise late in the development schedule when major components are exchanged for others for example the vehicle refinement was successfully performed for a sixcylinder variant but the four-cylinder model is discovered to have a major booming noise. If initial surveys indicate a structure-borne problem, transfer path analysis is used to identify the most effective locations for palliative treatment such as a softer engine mount rather than major structural redesign. 2. NVH Refinement The target levels for vehicles interior noise have been substantially lowered over the years. The application of balancing shafts, the use of six cylinder engines and better design of driveline suspension means that the primary sources of disturbing noise have been well understood and controlled. Consequently, the contribution of other noise sources has become the limiting factor in the drive to improve acoustic comfort. These secondary sources transmit energy either through structure-borne paths (gear meshing, wheel inputs, engine compartment vibration...), or airborne paths (tire noise, aerodynamic excitation, intake noise, engine compartment radiation...) or a combination of both. As no single source dominates the overall noise level, further refinement requires a more sophisticated strategy than the test-analyze-fix approaches of only a few years ago. In-depth transfer path analysis is used to identify every possible path and to provide alternatives for the development team. The enormous amount of TPA results should be easily accessible and clearly organized. Dedicated color displays show the amplitude of the partial contributions for all selected paths as a function of rpm or frequency. Engineers can quickly visualize the relative importance of the different paths. Figure 3 gives an example of the 4D view on TPA data. 1. Contribution per path and per case (i.e. an order) for a specific operational condition (frequency or RPM) 2. Contribution over the entire frequency or RPM range per path for a specific case 3. Contribution over the entire frequency or RPM range per case for a specific path 4. Vector display showing the amplitudes and phase of each path for a specific case at a specific frequency or RPM Technical info issued by: Siemens. 3

4 Figure 3: A contribution display (also called 4D display) gives a clear overview of the most contributing paths. Display 1, 2 and 3 show the most contributing paths in the model. Display 4 reveals which paths have cancelling effects due to opposite phases and what would be the effect of the modification of a contributing path. V. What-if engineering simulation The various TPA techniques can be expanded to support whatif scenarios, whereby loads and/or transfer path can be interactively modified and visually evaluated in real-time. Multiple modifications can be compared against each other. This greatly enhances the target setting process. One strong point of the method is that it lets engineers perform simulation using only test-based data. The following example shows results of a what-if simulation using the results of a transfer path analysis performed on a vehicle. Figure 9 displays the time signal of the sound pressure level in the car s interior. The vehicle s interior is described in the analysis as the target position. In figure 9 the red curve expresses levels before modification. This figure clearly shows that during acceleration high levels can be observed at specific operating conditions. Figure 9: The time domain data clearly shows the impact of the modification. Technical info issued by: Siemens. 4

5 The TPA analysis revealed that a specific path was responsible for these high levels. This path becomes very visible on the contribution map (figure 10). VI. Conclusion TPA is used to identify and assess structure-borne and airborne energy transfer routes from the excitation source to a given receiver location. Transfer path analysis sets out to quantify the various sources and their paths and figure out which ones are important, which ones contribute to the noise issues and which ones cancel each other out. Integrating all of this is a software tool is not an easy task on the other hand. A TPA software tool needs to be designed to manage a large amount of data collected during measurement campaigns in a convenient and efficient way. Using embedded documentation, transfer functions and operational data need to be automatically sorted through easy transfer path model definition. Such automated procedures eliminate data handling errors and make the process as productive as possible. Figure 10: The display shows most contributing paths. A TPA software tool should focus on ease-of-use and productivity. Engineers appreciate a workflow-oriented GUI, a powerful data management that continuously checks data and minimizes translation issues and operator errors. Using what-if engineering capability, the characteristics of that path can be modified (edited) to simulate the effect of a selected design modification, resulting in the contribution map in figure 10. In figure 9, the green curve finally represents the time signal of the sound pressure levels as a result of the simulated modification. The critical booming noise has been clearly reduced. In this analysis, the time signals can be used as well for objective and subjective evaluation of the problem and proposed solutions. A TPA software tool should help users perform data processing and results interpretation fast and efficiently. The enormous amount of TPA results should be easily accessibly and clearly organized. For each operational condition and transfer path, the operational forces should be quickly retrievable and accessible. To quickly visualize the relative importance of the different paths, dedicated color displays should allow showing amplitudes of the partial contributions for all selected paths as a function of RPM or frequency. A TPA software tool should helps users analyze the interior acoustic response both objectively and subjectively and identify the disturbing spectral components or even the lack of masking constituents. For those frequencies, the operating and laboratory data are combined to quantify the contributions of the various sources and their paths. Once the sources and their paths have been quantified and modeled, it is a relatively straightforward design task to optimize the system. The various TPA techniques could be expanded to support what-if scenarios, whereby loads and/or transfer paths are interactively modified and visually evaluated in real-time. Multiple modifications could be compared against each other, which would greatly enhance the target setting process. Figure 11: As an effect of the modification, the critical booming noise has been reduced. LMS Test.Lab Transfer Path Analysis is such a solution, offering a complete suite of tools for classical TPA, OPA, LMS OPAX, Multi-reference TPA, and Time-domain TPA. This software suite is based on what LMS has learned in the past 20 years of pioneering TPA tools. Extensive consolidated hands-on experience, especially also in engineering consultancy projects with many major global industry leaders across the world has Technical info issued by: Siemens. 5

6 been translated into innovative extensions that allow engineers solve mission-critical noise and vibration issues Siemens Product Lifecycle Management Software Inc. Siemens and the Siemens logo are registered trademarks of Siemens AG. LMS, LMS Imagine.Lab, LMS Imagine.Lab Amesim, LMS Virtual.Lab, LMS Samtech, LMS Samtech Caesam, LMS Samtech Samcef, LMS Test.Lab, LMS Soundbrush, LMS Smart, and LMS SCA- DAS are trademarks or registered trademarks of LMS International N.V. or any of its affiliates. All other trademarks, registered trademarks or service marks belong to their respective holders. Technical info issued by: Siemens. 6